Recently, with the propagation of personal computors, word processors, etc., magnetic disks have been required to have a higher recording density.
In particular, a magnetic recording medium having a magnetic recording layer mainly composed of a ferromagnetic metal powder and a binder (hereinafter referred to as a metal powder type magnetic recording medium) is excellent in magnetic characteristics such as coercive force (Hc), saturation magnetic flux density (Bm), etc., and, thus, has been expected to be used as a magnetic recording medium for high recording density. Also, the metal powder type magnetic recording medium has been practically used as a recording media for 8 mm video tapes and video floppy disks.
For further increasing the recording density of the metal powder type magnetic recording medium, various means for pulverizing a ferromagnetic metal powder, smoothing the magnetic recording layer, and uniformly dispersing a ferromagnetic metal powder in the magnetic recording layer have been applied.
In these means, the means of pulverizing a ferromagnetic metal powder is particularly effective and there is a tendency toward using a ferromagnetic metal powder having a large specific surface area.
However, when the ferromagnetic powder is pulverized in the aforesaid metal powder type magnetic recording medium, the surface of the magnetic layer thereof is very smooth, which results in increasing the friction coefficient of the magnetic layer and each part in a magnetic recording and reproducing apparatus during running of the magnetic recording medium.
Furthermore, in a magnetic disk, various troubles on the running durability, such as the occurrences of stick slip, sticking of the magnetic layer onto a magnetic head, etc., which have never occurred in the case of using conventional .gamma.-iron oxide and Co-modified iron oxide, are caused.
Hitherto, various methods for improving the running durability of magnetic recording media have been proposed.
For example, there are proposed methods of adding abrasives into magnetic layers. That is, there are a method of adding .alpha.-type ferric oxide as disclosed in JP-A-58-159227 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), a method of adding Al.sub.2 O.sub.3 as disclosed in JP-A-58-171721, a method of adding particles having a mean particle size of not more than 0.2 .mu.m and a Mohs' hardness of at least 6 as disclosed in JP-A-61-289528, etc. However, when a sufficient amount of each abrasive for improving the running durability of the magnetic recording medium is added according to these methods, the abrasion of the magnetic head is accelerated and also the surface property of the magnetic layer is reduced to lower the electromagnetic characteristics thereof.
Also, a method of using an abrasive and a fatty acid ester (JP-A-58-189826) and a method of using carbon black and a silicone oil (JP-A-62-195730) are proposed.
Furthermore, as a method of adding a lubricant into a magnetic layer, there are a method of adding a polyethylene glycol ester (JP-A-57-3225), a method of adding a ketone compound (JP-A-57-158026), a method of adding a glycerol ester of a fatty acid and a sorbitan ester of a fatty acid (JP-A-57-158027), a method of adding a fatty acid and a fatty acid ester (JP-A-58-130435 and JP-A-60-5420), and a method of adding a fatty acid alkyl ester and an fatty acid alkoxy ester (JP-A61-229234 and JP-A-61-230624). However, in these methods, the control of the addition amount of the lubricant(s) is difficult and if the lubricant is added to an extent of obtaining the lubricating effect, the mechanical strength of the magnetic layer is weakened to reduce the running durability of the magnetic recording medium and further cause sticking of the layer onto a magnetic head.
Furthermore, although the problems about the running durability of a magnetic recording medium using a fine ferromagnetic metal powder may be solved to some extent by the aforesaid methods, the extent of the improvement by these methods is yet insufficient and also these methods are sometimes accompanied by the reduction of other characteristics of the magnetic recording media.
In particular, when the on-off of a magnetic head onto a magnetic layer is repeated many times as in the case of a floppy disk, the friction coefficient of the magnetic layer is increased during running thereof to cause the vibration of a magnetic head by stick slip thereof, thereby the output of the magnetic recording medium becomes unstable and also to cause sticking of the magnetic layer onto a magnetic head, thereby running of the magnetic recording medium becomes inferior. However, it is very difficult to solve these problems.
Moreover, when a ferromagnetic metal powder is used for a floppy disk as the ferromagnetic powder thereof, there is a problem that the magnetic layer is injured by rubbing against the liner of a jacket for the floppy disk, which is not so serious in the case of using a conventional iron oxide-series ferromagnetic powder. That is, the magnetic layer of a floppy disk is injured by rubbing against the liner composed of a nonwoven fabric during running of the floppy disk, thereby drop out is liable to occur during the use of the floppy disk. This problem is liable to become more severe with reduction of the particle sizes of the ferromagnetic metal powder. An effective means for solving the problems occurring in the floppy disk using a ferromagnetic metal powder has not yet been proposed.